There is a widely known navigation device that detects the location of a traveling vehicle using a global positioning system (GPS) receiver and identifies the vehicle location on a map by collating the detected vehicle location with map data stored in a database.
The map data used by such a navigation device is generated by sampling not points on a road at fixed intervals on, for example, a commercially available road map. Then, the node points are extracted and two-dimensional coordinate information about the node points is stored in a storage medium. However, it has been difficult to provide a low-priced car navigation device because the introduction of a map database needs high cost. In view of such circumstances, there is a proposed device that creates map data based on routes actually traveled by a vehicle (for example, refer to Patent Literature 1).
When an electronic road map described in Patent Literature 1 is used, information about node points stored in a storage medium is examined to extract three neighboring node points, and a curvature at each node is obtained by calculating an approximate expression of a circle that passes through the three neighboring node points. Further, an interval including the nodes is approximated and expressed by using a straight line, an arc, or a clothoid curve (a curve changing from a straight line to an arc) based on the value of the calculated curvature. Thus, a highly accurate road map can be displayed using a limited amount of data.
When a vehicle is about to travel on a sharp curve, the shape of the curve should preferably be extracted with higher accuracy in order to enable a driver of the vehicle to properly change the speed of the vehicle with a natural feeling wherever possible. Currently popular vehicular navigation maps are designed to search for a route to a destination and provide route guidance to the destination. However, the currently popular vehicular navigation maps are not accurate enough to provide a driving control yet.
Under the above circumstances, the applicant of the present application proposed a technology that is described in Japanese Patent Application No. 2011-149514 in order to restrict a decrease in accuracy of created map data caused by a measurement error of a location sensor. This technology enables a generation of accurate map information even if a location sensor has a measurement error.
The technology described in Japanese Patent Application No. 2011-149514 can further be improved. More specifically, a steady interval (an arc interval having a constant curvature) existing on an actual road is not found in an actually traveled path. Therefore, the length of the steady interval cannot be accurately quantified even when the above-mentioned technology is used. In such a case, it is difficult to determine an interval that needs a speed control.